1. Field of the Invention
The present invention relates to bipolar transistor structures, but more specifically, the present invention relates to homojunction and heterojunction trenched bipolar transistor structures.
2. Description of the Prior Art
Silicon bipolar transistors are produced by a highly evolved, long established and well known technology. FIG. 1 is a generalized isometric/schematic cross-section view of a conventional homojunction silicon bipolar transistor (SBT) structure 10 which can be fabricated by this technology. The conventional homojunction SBT structure 10 comprises a base region 12 sandwiched between an emitter region 14 and a collector region 16. An active region 18 of the conventional homojunction SBT structure 10 is that area directly beneath the emitter region 14 as bounded by the dotted lines 20. The active region 18 is approximately equal to the area of the emitter region 14, which is of width w per unit length thereof. As shown, the depth d of the emitter region 14 is usually smaller relative to its width w. Thus, since the dimensions a and a' are greater than the dimension b of the base region 12, beneficial contribution of the side areas, i.e., 2 d per unit length, to the active operation of the emitter region 14 is negligible. In fact, none of the other portions of the conventional homojunction SBT struction 10 beneficially contribute to its active operation. For example, the base-to-collector junction 22, which interfaces the base region 12 and collector region 16, creates an unwanted base-to-collector parasitic capacitance. This unwanted base-to-collector parasitic capacitance lies in the horizontal and vertical planes of the base-to-collector junction 22 to the right and to the left of the dotted lines 20, outside the active region 18, i.e., the parasitic region.
Still referring to the conventional homojunction SBT structure of FIG. 1, to minimize the unwanted base-to-collector parasitic capacitance, the dimensions a, a' and b of the base region 12 should be equal. However, as illustrated in FIG. 1, in practical application, the dimensions a and a' of the base region 12 must be of sufficient width to permit metallization of the surfaces of base ohmic contacts (not shown). In addition, the width w of the emitter region 14, which translates into the width of the active region 18, and, accordingly, the beneficial width of the base-to-collector junction 22, must be kept small to minimize the effective base resistance. Thus, in actual practice, the parasitic region typically exceeds that of the active region 18. This results in the dimensions a and a' being much larger than the dimension b. Hence, the ideal conventional homojunction SBT structure 1 and its above mentioned practical counterpart does not utilize its volume in the most effective and efficient manner.